Because I don't have to know T-SQL inside out and upside down in order to use R for something that SQL won't even do.
There's no question that R brings some fantastic stuff to the table and that there's a lot of stuff that it can do that T-SQL can't do never mind being super easy to do in R and I'm not challenging that. What I am challenging is the use of R, Powershell, SQL CLR, VBS, etc, etc, etc to do things that are actually quite easy to do in T-SQL that people just don't seem to take the time to learn because they think that SQL Server is just a place to store data.
My favorite example is when 2005 came out and the use of SQL CLR was all the rage. To make a much longer story shorter, a "developer" brought me an SQL CLR to deploy to production and I refused. He stormed out of the room before I could complete the word "No" and explain and he took it to management and I was called out on the carpet because it was supposedly a showstopper holding up a project. What did the SQL CLR do that was so critical. It calculated a MODULO. You know... the one that uses the "%" operator in T-SQL. <headdesk><major facepalm>
That's what I'm talking about.
But that doesn't require 'knowing T-SQL very well'. You're talking there about knowing the basics of the language.
If you use R because you don't know the first thing about T-SQL, then you're not a T-SQL developer using R for something they don't know is in T-SQL, you're an R developer.
And, if I may be blunt, using "developer" to refer to someone who develops predominantly/only in other languages is frankly insulting. Sure, the guy may well have had an attitude problem and should have asked if the C# was necessary before he wrote it, that doesn't mean he's not a developer.
I work with a whole bunch of data professionals who don't know the first thing about T-SQL, or the base SQL language in general. That doesn't make them inferior.
It was meant as a strong insult to that particular person because of his poor attitude.
Agreed on the "very well" comment. To be sure, I'm not talking about the "Black Arts" that a lot of us know how to do. I'm talking about what most of us would consider to be "the basics", like how to do a 3 table join, when to use an inner join or an outer join, what many of the functions in the language do, a bit about how to write a SARGable query, and maybe even how to use APPLY